Polytetrafluoroethylene (PTFE) fiber is a desirable commercial form of PTFE, however, manufacture of continuous strong fibers from PTFE resin is a difficult process. Conventional fiber forming techniques used in production of fibers from other polymeric materials are of little use in producing PTFE fibers.
Direct extrusion of a PTFE melt through spinnerets or multiple fine holes in a die is impractical due to the extremely high melt viscosity of the polymer. Similarly, a PTFE coagulated dispersion or fine powder in which an extrusion aid has been mixed also exhibits extremely high viscosity upon direct extrusion.
To overcome the difficulties of extruding high viscosity PTFE resin in a fiber form, a process was developed as taught in U.S. Pat. No. 2,772,444 to Burrows, et al. In this process, an aqueous dispersion of PTFE resin is mixed with cellulose xanthate and extruded by conventional fiber forming techniques used to make viscose fibers. The extruded PTFE/cellulose xanthate mix is subsequently heated to high temperatures so to thermally decompose the cellulose xanthate while at the same time heating the residual PTFE above its crystalline melt point thereby forming coherent PTFE fibers. The product of this process is a multiple filament fiber of low tenacity and having a brown color. The brown color can be removed and the multiple filament fiber returned to the usual white color of other unpigmented PTFE products though the use of a bleaching process; however, bleached fibers exhibit approximately 50% of the tenacity of unbleached fibers.
Later, as taught in U.S. Pat. Nos. 3,953,566, 3,962,153 and 4,187,390 all to Gore, a form of PTFE combining high strength and high porosity was made by expanding PTFE shapes. In U.S. Pat. No. 3,962,153, a PTFE fiber is produced through the rapid expansion of a shaped PTFE rod. This process is not desirable for the commercial production of fibers since fibers produced by this process are not made in continuous lengths but rather are limited by the length of the rod and the ability to expand the rod.
In U.S. Pat. Nos. 3,953,566 and 4,187,390, a continuous sheet of expanded microporous PTFE has been made by expanding PTFE in tape or sheet form. This sheet of expanded microporous PTFE is subsequently slit to form a slit film fiber in continuous length as shown in FIG. 1. Slit film fibers of expanded microporous PTFE are much stronger, with tenacities 21/2 to 5 times or more than those fibers of the same denier made by the process of U.S. Pat. No. 2,772,444. Furthermore, slit film fibers of expanded microporous PTFE exhibit low shrinkage and high modulus of elasticity at elevated temperatures.
However, slit film fibers of expanded microporous PTFE have a tendency to fibrillate or fray at the slit edges of the fiber. Also slit film fibers of expanded microporous PTFE are not round but are rectangular in cross-section. Therefore, to obtain a more uniform appearance in slit film fibers of expanded microporous PTFE slit film fibers of expanded microporous PTFE are twisted along their longitudinal axis producing twisted fibers. Twisted fibers give the illusion of appearing more round in cross-section than untwisted fibers by presenting a mixture of sides of the rectangular cross-section of the fibers to the viewer which is especially desirable when slit film fibers of expanded microporous PTFE are used in the production of knitted or woven fabrics. The fibers may be twisted to the extent that tubular structures are produced. It is desirable that fibers have uniform cross-sections along their lengths so that fabrics produced from these fibers contain interstices of an uniform and predictable nature. Fabrics containing interstices of a uniform and predictable nature when used as a filter element exhibit a constant filtration efficiency across the surface of the fabric. However, slit film fibers of expanded microporous PTFE are difficult to produce with a consistent twist along the length of the fibers, thereby resulting in fibers with non-uniform cross sections along their length. Fibers having inconsistent twists, as well as fabrics produced from these fibers, are considered flawed when visually inspected, and are of limited utility as a filter element.
It is to the provision of continuous PTFE fibers with substantially round profiles that this invention is directed.